vi The Solar System and Universe 87 



and to show the movements of planets. They are so like 

 stars that the name Asteroid (star-like) is usually given 

 them. No. 311 was discovered on nth June 1891. 

 These minor planets are all very small, the largest being 

 probably only 300 miles in diameter; the orbits of some 

 are very long ellipses, and lie far out of the plane of the 

 ecliptic (see 132). 



130. Outer Planets. Beyond the asteroid ring the 

 giants of the solar system, each attended by a train of 

 satellites, rotate with amazing speed, and are surrounded 

 by thick atmospheres loaded with heavy clouds. Jupiter, 

 the largest of all, with four satellites, has a temperature so 

 high that dense layers of cloud, arranged in belts parallel 

 to the equator by its rapid rotation, completely obscure the 

 body of the planet. The spectrum of its light shows some 

 dark bands which are not due to reflected sunlight, and it is 

 generally assumed that Jupiter is only now cooling down 

 from being a self-luminous body. Saturn, although some- 

 what smaller, is unique in being accompanied by a series 

 of rings or thin flat discs surrounding its globe parallel to 

 the equator, and reflecting sunlight like the planet itself. 

 These rings can only be accounted for on the assumption 

 that they are composed of orderly crowds of innumerable 

 minute satellites. Outside the rings there are eight separate 

 satellites of various sizes, one being larger than the Moon. 



131. Uranus and Neptune. Uranus has been known 

 as a planet since 1781, when it was discovered by Herschel. 

 One astronomer had observed it previously twelve times, 

 and only the careless way in which he kept his notes pre- 

 vented him from recognising it as a new member of the 

 solar system. This remote body is remarkable for its four 

 satellites revolving in apparently circular orbits in a plane at 

 right angles to that of the planet's orbit, and from east to 

 west, whereas the satellites of all planets nearer the Sun 

 revolve, like the Moon, from west to east. The movements 

 of planets in their orbits under solar attraction is calculated 

 from Kepler's Laws ( 109), but allowance has always 

 to be made for the perturbations or deviations produced by 

 the attraction of other planets. After all possible allowances 



